1. Field of the Invention
The present invention relates to a single-mode optical fiber usable as a transmission line in optical communications and the like; and, in particular, to a dispersion-shifted optical fiber suitable for wavelength division multiplexing (WDM) transmission.
2. Related Background Art
In general, a WDM transmission system using optical fiber networks is a system enabling long-distance, large-capacity optical data communications, and is constituted by a transmitter/receiver for transmitting and receiving WDM signals of a plurality of wavelengths (light signals), an optical amplifier such as an optical fiber amplifier for amplifying the WDM signals, an optical fiber which is a transmission medium, and the like. In such a WDM transmission system, the wavelength band that can optically be amplified in the optical amplifier is from 1530 nm to 1560 nm, whereas the low-loss wavelength band in the optical fiber is from 1400 nm to 1700 nm. As a consequence, the wavelength band utilizable as the WDM signals in the conventional WDM transmission system has substantially been limited to a width of about 30 nm from 1530 nm to 1560 nm.
The amplification of WDM signals by the optical amplifier increases the optical power of each light signal in the optical fiber acting as the transmission medium, thereby causing nonlinear phenomena such as four-wave mixing, self phase modulation, modulation instability, and the like. In particular, the four-wave mixing causes power variations among the individual signal components, whereas the self phase modulation distorts the pulse waveform of each light signal upon an interaction with the chromatic aberration of the optical fiber (hereinafter referred to as dispersion), whereby the occurrence of such nonlinear phenomena limits the normal transmission of light signal.
The inventors have studied the case where a conventional dispersion-shifted optical fiber is employed in a WDM transmission system and, as a result, have found problems as follows.
Namely, for effectively s Oppressing the four-wave mixing, it is preferred that the wavelength of each light signal be different from the zero-dispersion wavelength of the optical fiber. For effectively suppressing the self phase modulation, on the other hand, it is preferred that the absolute value of dispersion value of the optical fiber with respect to each light signal be not so large.
When the four-wave mixing and the self phase modulation are compared with each other, the distortion in pulse waveform of each light signal caused by the self phase modulation can be alleviated to a certain extent by a dispersion-compensating technique in which a dispersion-compensating optical fiber (having a dispersion characteristic with a polarity opposite to that of the dispersion value of the optical fiber acting as the transmission medium) is inserted in the optical transmission line through which each light signal propagates, so that the dispersion value of the optical transmission line as a whole becomes nearly zero. By contrast, no technique has been known for compensating for the crosstalk between individual light signals caused by the four-wave mixing. Therefore, as compared with the self phase modulation, it is more important to suppress the four-wave mixing.
In view of the increase in noise components caused by the modulation instability, on the other hand, it is preferred that the zero-dispersion wavelength be set on the longer wavelength side from the wavelength band of each light signal. Further, letting N2 be the nonlinear refractive index of the optical fiber, Aeff be the effective area thereof, P be the power of light the propagating therethrough, and Leff be the effective length of the optical fiber, the amount of occurrence of nonlinear phenomena in the optical fiber is given by the following expression (1):
N2xc2x7Pxc2x7Leff|Aeffxc2x7xe2x80x83xe2x80x83(1)
Among these parameters, the nonlinear refractive index N2 is determined by the material of the optical fiber, whereby it is necessary for the effective area Aeff of the optical fiber to increase in order to reduce the amount of occurrence of nonlinear phenomena.
Here, as shown in Japanese Patent Application Laid-Open No. HEI 8-248251 (EP 0 724 171 A2), the above-mentioned effective area Aeff is given by the following expression (2):                               A          eff                =                  2          ⁢                      xe2x80x83                    ⁢                                                    π                ⁡                                  (                                                            ∫                      0                      ∞                                        ⁢                                                                  E                        2                                            ⁢                      r                      ⁢                                              ⅆ                        r                                                                              )                                            2                        /                          (                                                ∫                  0                  ∞                                ⁢                                                      E                    4                                    ⁢                  r                  ⁢                                      ⅆ                    r                                                              )                                                          (        2        )            
where E is the electric field accompanying the propagating light, and r is the radial distance from the center of the core region.
In the conventional WDM transmission system, as a result of the foregoing studies, it is preferred that the zero-dispersion wavelength of the optical fiber be restricted to the range of 1560 nm to 1600 nm, and that its effective area Aeff be 50 xcexcm2 or more. Further, for suppressing the increase in loss upon cabling the optical fiber, it is preferred that its bending loss be smaller, whereby its cutoff wavelength must be set to an appropriate value.
In the conventional WDM transmission system, the channel spacing between the individual light signals included in the WDM signals is about 1 nm, whereby the actual multiplicity has been limited to about 30 waves. For enhancing the transmission capacity, however, it is desirable that the wavelength multiplicity be increased. In this case, while a method of narrowing the channel spacing and a method of enlarging the wavelength bandwidth can be considered, the latter is preferred in view of the above-mentioned suppression of four-wave mixing.
On the other hand, the amplification wavelength band of the optical fiber amplifier has been expanding along with the advance in technology, thereby making it possible to amplify the WDM signals in a wider wavelength band of 1530 nm to 1610 nm (see, for example, A. Mori, et al., xe2x80x9c1.5 xcexcm Broadband Amplification by Tellurite-Band EDFAs,xe2x80x9d OFC ""97, PD1). In contrast, the zero-dispersion wavelength of the conventional dispersion-shifted optical fiber lies within the range of 1560nm to 1600 nm as mentioned above. Therefore, in a WDM transmission system employing the optical fiber amplifier having thus expanded amplification wavelength band and the conventional dispersion-shifted optical fiber, there is a possibility that the zero-dispersion wavelength of the conventional dispersion-shifted optical fiber may lie within the wavelength band that can be amplified by the optical fiber amplifier (the signal wavelength band of WDM signals), so that the four-wave mixing may occur strongly, whereby the WDM signals may not be transmitted normally.
In order to overcome the problems such as those mentioned above, it is an object of the present invention to provide a dispersion-shifted optical fiber suitable for an optical transmission line in which an optical fiber amplifier having an expanded amplification band is installed.
The dispersion-shifted optical fiber according to the present invention is a single-mode optical fiber comprising a core region extending along a predetermined axis, and a cladding region provided on the outer periphery of the core region; wherein a zero-dispersion wavelength is set within a range of 1610 nm or more but 1750 nm or less, preferably 1610 nm or more but 1670 nm or less, so that the optical fiber is employable in a WDM transmission system including an optical amplifier having an expanded amplification wavelength band. Particularly, in order to suppress the occurrence of nonlinear phenomena across a signal wavelength band by slightly generating a dispersion, the zero-dispersion wavelength is preferably set within a range of 1640 nm or more but 1750 nm or less, further preferably 1640 nm or more but 1670 nm or less. Also, the dispersion-shifted optical fiber according to the present invention has a cutoff wavelength of 1.1 xcexcm or more at a length of 2 m and has, with respect to light having a wavelength of 1550 nm, which is light in a signal wavelength band, an effective area of 45 xcexcm2 or more, preferably 50 xcexcm2 or more, further preferably 70 xcexcm2 or more, and a dispersion slope of 0.15 ps/nm2/km or less.
Since the dispersion-shifted optical fiber according to the present invention has an effective area of 45 xcexcm2 or more, preferably 50 xcexcm2 or more, further preferably 70 xcexcm2 or more, it can suppress nonlinear phenomena by the same extent as that of the conventional dispersion-shifted optical fiber or more. While the absolute value of dispersion value in a typical optical fiber having a zero-dispersion wavelength near 1.3 xcexcm becomes about 20 ps/nm/km when employed in WDM transmission in a 1.5-xcexcm wavelength band, a certain degree of transmission quality can be guaranteed by utilization of a dispersion compensating technique such as that mentioned above. Therefore, the dispersion-shifted optical fiber according to the present invention has a dispersion slope of 0.15 ps/nm2/ km or less with respect to light a wavelength of 1550 nm so that the absolute value of dispersion value becomes 20 ps/nm/km or less. Further, in accordance with the specification of Bellcore, it is necessary that the bending loss upon bending at a diameter of 32 mm be 0.5 dB/turn or less. Therefore, the cutoff wavelength at a reference length of 2 m (measured by a method defined by ITU international standard) in the dispersion-shifted optical fiber according to the present invention is 1.1 xcexcm or more so that the bending loss at a diameter of 32mm is 0.5 dB/turn or less. Here, the dispersion slope refers to the gradient of a graph indicating the wavelength dependence of dispersion value (see, for example, FIG. 5A).
This dispersion-shifted optical fiber is employable in a WDM transmission system together with an optical amplifier such as an optical fiber amplifier whose amplification wavelength band is expanded, for example, to the range of 1530 nm to 1610 nm. Namely, even in the case where the individual light signals included in the wavelength band of 1530 nm to 1610 nm that can be amplified by the optical fiber amplifier propagate through the dispersion-shifted optical fiber, the occurrence of nonlinear phenomena such as four-wave mixing, self phase modulation, and the like is suppressed. Also, since the dispersion-shifted optical fiber has an excellent bending characteristic, it can realize high-quality optical transmission.
For yielding various characteristics such as those mentioned above, the dispersion-shifted optical fiber according to the present invention can be realized by various refractive index profiles.
For example, a desirable refractive index profile can be realized when the above-mentioned core region is constituted by a center core having a predetermined refractive index and an outer core provided on the outer periphery of the center core and having a refractive index lower than that of the center core. In this configuration, the maximum value of relative refractive index difference of the center core with respect to a reference area in the cladding region is preferably 0.9% or more in view of its relationship with the zero dispersion wavelength.
Also, a desirable refractive index profile can be realized when the above-mentioned core region is constituted by a center core having a predetermined refractive index; an intermediate core provided on the outer periphery of the center core and having a refractive index lower than that of the center core; and, an outer core provided on the outer periphery of the intermediate core and having a refractive index higher than that of the intermediate core. In this configuration, the maximum value of relative refractive index difference of the center core with respect to the reference area in the cladding region is preferably 0.6% or more in view of its relationship with the zero dispersion wavelength.
Further, a desirable refractive index profile yielding the above-mentioned various characteristics can be realized when the above-mentioned core region is constituted by a center core having a predetermined refractive index and an outer core provided on the outer periphery of the center core and having a refractive index higher than that of the center core. In this configuration, the maximum value of relative refractive index difference of the outer core with respect to the reference area in the cladding region is preferably 0.8% or more in view of its relationship with the zero dispersion wavelength.
Here, the above-mentioned refractive index profile is represented by the relative refractive index difference xcex94 ni given by the following expression (3):
xcex94ni=(nixe2x88x92ncd)/ncdxe2x80x83xe2x80x83(3)
where ncd is the refractive index of the reference area (SiO2) in the cladding region, and ni is the refractive index of each part i constituting the core region. Hence,the relative refractive index difference xcex94ni is represented with reference to the average refractive index ncd of the reference area in the cladding region. In this specification, the relative refractive index difference is expressed in terms of percentage, and regions having a negative refractive index refer to those having a refractive index lower than that of the reference area.
Further, in the dispersion-shifted optical fiber according to the present invention, the cladding region can comprise a depressed cladding structure constituted by an inner cladding provided on the outer periphery of the core region, and an outer cladding provided on the outer periphery of the inner cladding and having a refractive index higher than that of the inner cladding. A combination of this depressed cladding structure and any of the core region structures mentioned above can realize a desirable refractive index profile. In the case of the depressed cladding structure, the above-mentioned relative refractive index difference is given while the above-mentioned outer cladding is employed as the reference area.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.